1. Field of the Invention
The present invention relates to a semiconductor device and a method for fabricating the same. More specifically, the present invention relates to a semiconductor device comprising trench-isolated transistors and a method for fabricating the same.
2. Description of the Background Art
Semiconductor devices provided with a plurality of transistors isolated by trenches have been widely known, and one such device is disclosed in xe2x80x9cA Shallow-Trench-Isolation Flash Memory Technology with a Source-Bias Programming Methodxe2x80x9d IEDM, pp.177-180, 1996.
FIG. 11 shows a cross sectional view of the prior art semiconductor device disclosed in the above-mentioned literature. Referring to FIG. 11, the prior art semiconductor device has a plurality of trenches 101h on a silicon substrate 101. Trenches 101h are defined by a surface 101s of silicon substrate 101. Surface 101s contains an oxide layer 111, and a silicon oxide film 112 is formed on surface 101s. A polysilicon film 119 is filled into trenches 101h in such a manner that polysilicon film 119 is in contact with silicon oxide film 112, and an oxide layer 117 is so formed as to cover the surface of polysilicon film 119.
Between adjacent trenches 101h, floating gate electrodes 122 are arranged on silicon substrate 101 with a gate insulator film 121 disposed therebetween. Floating gate electrodes 122 are composed of a bottom conductor film 122a and a top conductor film 122b. 
A silicon oxide film 123 is so laid as to be in contact with the side walls of lower conductor layer 122a and with gate insulator film 121. On silicon oxide film 123 is a side-wall insulator layer 124 made of a silicon nitride film.
Adjacent floating gates 122 are isolated from each other by trenches 101h formed therebetween.
A control gate electrode 131 is formed on floating gate electrodes 122 with a dielectric film 125 disposed therebetween. Control gate electrode 131 is extended from the right to the left side of the drawing.
A method for fabricating the semiconductor device of FIG. 11 will be described as follows. FIGS. 12 to 15 show cross sectional views depicting a method for fabricating the semiconductor device of FIG. 11. Referring to FIG. 12, gate insulator film 121 is formed on the surface of silicon substrate 101. A polysilicon film, a silicon oxide film, and a silicon nitride film are formed on gate insulator film 121 in this order. A patterned resist pattern is provided on the silicon nitride film. Then, the silicon nitride film, the silicon oxide film, and the polysilicon film are etched in accordance with the resist pattern. As a result, a silicon nitride film 127, a silicon oxide film 128, and lower conductor layer 122a are completed. While using silicon nitride film 127 and lower conductor layer 122a as a mask, impurity ions are implanted into silicon substrate 101, thereby forming impurity regions 116 as source and drain regions.
Referring to FIG. 13, silicon oxide film 123 and a silicon nitride film are accumulated on the surface of silicon substrate 101, and the entire surface of silicon substrate 101 is etched back so as to form side-wall insulator layer 124 and silicon oxide film 123. While using silicon nitride film 127 and side-wall insulator layer 124 as a mask, the entire surface of silicon substrate 101 is etched back so as to form trenches 101h. The surfaces of trenches 101h are oxidized to form an oxide layer 111.
Referring to FIG. 14, silicon oxide film 112 is formed by CVD (chemical vapor deposition), and polysilicon film 119 is accumulated on silicon oxide film 112. The entire surface of semiconductor substrate 1 is, etched back so as to expose silicon nitride film 127.
Referring to FIG. 15, the surface of polysilicon film 119 is oxidized to form oxide layer 117. Then, silicon nitride film 127 is eliminated.
Referring to FIG. 11, after the surface of lower conductor layer 122a is washed, a polysilicon film is formed. The polysilicon film is etched to form upper conductor layer 122b. On upper conductor layer 122b is formed dielectric film 125 made of an ONO (oxide nitride oxide) film composed of a silicon oxide film, a silicon nitride film, and another silicon oxide film. Control gate electrode 131 is arranged on dielectric film 125, thereby completing the semiconductor device shown in FIG. 11.
The problems of the aforementioned prior art semiconductor device will be described as follows with reference to the drawings.
FIG. 16 shows a cross sectional view depicting a problem of the prior art semiconductor device. Referring to FIG. 16, in a prior art process, silicon substrate 101 is exposed in an oxidizing atmosphere while oxide layer 117 is being formed. During the exposure, an oxygen gas permeates through silicon oxide film 112 and oxide layer 111 so as to reach silicon substrate 101. Consequently, the portions of silicon substrate 101 that are in contact with surface 101s are oxidized to become oxide layer 135. Oxide layer 135, which is larger in volume than silicon, causes crystal defects in its vicinity. The occurrence of such crystal defects in the channel portions under floating gate electrodes 122 makes arsenic in impurity regions 116 be captured by the crystal defects, which shortens the substantial channel length. Supplying a potential difference across adjacent impurity regions in the transistors will cause a current to flow continuously due to the punch through between the source and the drain. This causes a problem that a selected memory cell transistor malfunctions, thereby deteriorating the reliability of the semiconductor device. There is a similar problem in forming oxide films of dielectric film 125, which is an ONO film.
FIG. 17 is a cross sectional view depicting another problem of the prior art semiconductor device. Referring to FIG. 17, electrons usually travel in the direction indicated by an arrow 142 between the source and the drain. However, some of the electrons traveling from the source to the drain proceed in the direction indicated by an arrow 143 and are captured by the trap level in gate insulator film 121, which is referred to as a hot electron phenomenon. The phenomenon causes the threshold voltage of the transistors to fluctuate, thereby decreasing the reliability of the semiconductor device.
The present invention, which has been contrived to solve the aforementioned problems, has an object of providing a highly reliable semiconductor device.
The semiconductor device of the present invention includes a semiconductor substrate, a gate electrode, a side-wall insulator layer, and a nitrogen-containing layer. The semiconductor substrate includes a first surface and a second surface which is adjacent to the first surface and defines trenches. The gate electrode has side wall, and is formed on the first surface of the semiconductor substrate with a gate insulator film interposed therebetween. The side-wall insulator layer is formed on the side wall and on a portion of the first surface. The nitrogen-containing layer is so formed as to extend from the portion of the semiconductor substrate that is in the vicinity of second surface to the portion of the semiconductor substrate that is in the vicinity of the interface between the side-wall insulator layer and the semiconductor substrate. The nitrogen-containing layer has a higher concentration of nitrogen than the first surface of the semiconductor substrate under the gate electrode.
In the semiconductor device thus structured, the nitrogen-containing layer is so formed as to extend from the portion of the semiconductor substrate that is in the vicinity of the second surface defining the trench to the portion of the semiconductor substrate that is in the vicinity of the interface between the side-wall insulator layer and the semiconductor substrate. In this region, the presence of nitrogen in the semiconductor substrate protects the substrate from being oxidized. Consequently, the semiconductor substrate is prevented from increasing in volume, which eliminates the possibility of the occurrence of a crystal defect. Hence, a punch through phenomenon is prevented so as to provide a highly reliable semiconductor device. In addition, the formation of the nitrogen-containing layer in the portion of the semiconductor substrate that is in the vicinity of the interface between the side-wall insulator layer and the semiconductor substrate decreases the density of a trap level in the insulator layer formed in this region. This decrease suppresses the capture of electrons, so as to obtain a highly reliable semiconductor device. Moreover, the nitrogen-containing layer has a higher concentration of nitrogen than the first surface of the semiconductor substrate that is under the gate electrode, so that the concentration of nitrogen is low in the channel regions under the gate electrode. As a result, a highly reliable semiconductor device capable of preventing the fluctuation of the threshold value can be provided.
It is preferable that the semiconductor device further includes an impurity region formed in the portion of the semiconductor substrate that is under the side-wall insulator layer. In this case, the occurrence of a punch through across the impurity regions can be prevented, so as to provide a highly reliable semiconductor device.
It is preferable that the semiconductor device further includes a buried insulator layer to fill the trenches.
It is also preferable that the semiconductor device further includes a control gate electrode formed on the gate electrode with a dielectric film interposed therebetween. In this case, a highly reliable nonvolatile semiconductor storage device can be provided.
It is preferable that the gate electrode includes a lower conductor layer so formed as to be in contact with the gate insulator film, and a upper conductor layer formed on the lower conductor layer opposite to face the control gate electrode, and the upper conductor layer has a larger width than the lower conductor layer. In this case, the upper conductor layer having a larger width than the lower conductor layer increases the area in which the upper conductor layer faces the control gate electrode. As a result, the capacity between the control gate electrode and the upper conductor layer increases, thereby providing a nonvolatile semiconductor storage device capable of operating with less voltage supplied to the control gate electrode.
It is preferable that the semiconductor device further includes an oxide layer formed on the second surface. In this case, the oxide layer formed on the second surface prevents the occurrence of an interface level.
It is preferable that the oxide layer is formed between the nitrogen-containing layer and the second surface.
The method for fabricating the semiconductor device of the present invention includes the steps of: forming gate electrode having side wall on a first surface of a semiconductor substrate with a gate insulator film interposed therebetween; forming a side-wall insulator layer on the side wall of the gate electrode and on a portion of the first surface; forming a trench defined by a second surface in the semiconductor substrate by etching the semiconductor substrate using the gate electrode and the side-wall insulator layer as a mask; and forming a nitrogen-containing layer extending from the portion of the semiconductor substrate that is in the vicinity of the second surface to the portion of the semiconductor substrate that is in the vicinity of the interface between the side-wall insulator layers and the semiconductor substrate by maintaining the semiconductor substrate in an atmosphere containing either nitrogen or a nitrogen compound.
According to the method for fabricating the semiconductor device including the above-described steps, the nitrogen-containing layer is so formed as to extend from the portion of the semiconductor substrate that is in the vicinity of the second surface defining the trenches to the portion of the semiconductor substrate that is in the vicinity of the interface between the side-wall insulator layer and the semiconductor substrate. In this region, the presence of nitrogen in the semiconductor substrate protects the substrate from being oxidized. Consequently, the semiconductor substrate is prevented from increasing in volume, which eliminates the possibility of the occurrence of a crystal defect. Hence, the punch through phenomenon is prevented so as to provide a highly reliable semiconductor device. In addition, the formation of the nitrogen-containing layer in the portion of the semiconductor substrate that is in the vicinity of the interface between the side-wall insulator layer and the semiconductor substrate decreases the density of the trap level in the insulator layer formed in this region. This decrease suppresses the capture of electrons, thereby realizing a highly reliable semiconductor device. Moreover, the nitrogen-containing layer has a higher concentration of nitrogen than the first surface of the semiconductor substrate that is under the gate electrodes, so that the concentration of nitrogen is low in the channel regions under the gate electrodes. As a result, a highly reliable semiconductor device capable of preventing a fluctuation in threshold value can be provided.
It is preferable that the method for fabricating the semiconductor device further including the step of forming an oxide layer by oxidizing the second surface before forming the nitrogen-containing layer. In this case, the oxide layer prevents the occurrence of an interface level in the second surface.
It is preferable that the method for fabricating the semiconductor device further including the step of forming impurity regions in the portions of the semiconductor substrate that are at both sides of each of the gate electrode by implanting an impurity into the semiconductor substrate while using the gate electrode as a mask after the formation of the gate electrode and before the formation of the side-wall insulator layer. In this case, the punch through phenomenon across the impurity regions is prevented so as to provide a highly reliable semiconductor device.
It is preferable that the method for fabricating the semiconductor device further includes the step of forming a buried insulator layer to fill the trench after the formation of the nitrogen-containing layer.
In addition, maintaining the semiconductor substrate in the atmosphere containing either nitrogen or the nitrogen compound includes maintaining the semiconductor substrate in an atmosphere of nitric oxide. In this case, the present of nitric oxide further decreases crystal defects in the side-wall insulator film.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.